Study on the alignment control of liquid crystalline molecules by photoisomerization reaction

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The photoisomerization of self-assembled monolayer films of 4-(4'-(methoxyphenylazo)phenoxy)undecane- 1-thiol (AZO 11) on evaporated gold films was investigated by means of Raman scattering and UV-visible and IR absorption measurements. The trans * cis photoisomerization of the self-assembled AZO11 molecules were restricted in comparison to the isomerization of AZO11 in solutions. Moreover, the trans-to-cis isomerization was more suppressed than the cis-to-trans isomerization, as expected from the molecular cross section of cis AZO11 that is larger than trans AZO11. On this basis we propose that cis-form samples are on the whole more suitable for cis trans photoisomerization in order to control the molecular alignment of liquid crystals. This indeed was verified in our experiments where thin films of a nematic liquid crystal (4-cyano-4'-pentylbiphenyl : 5CB) were deposited on the cis and trans AZO11 surface layers. IR reflection absorption spectroscopy revealed that 5CB molecules tend to align with their long axes perpendicular to the plane of the trans AZO11 layer. However, this alignment was affected by UV irradiation due to partial conversion to cis isomer. This fact demonstrates that liquid crystal alignment can be controlled by changes in the molecular shape on the substrate surface. Similar IR measurements were performed on gold film surfaces covered with a mixture of adsorbed AZO11 and hexadecanethiol (C_<16>H_<33>SH). in this case the limitation of AZO11 photoisomerization was relaxed and a larger change in the liquid crystal alignment was observed. The utility of IR transmission and Raman scattering measurements on silver island films was also demonstrated.